Input-output balanced filter

ABSTRACT

An input-output balanced filter has a greatly reduced size and excellent stable electric characteristics. The input-output balanced filter includes two LC bandpass filter circuit units. A first LC bandpass filter circuit unit has a circuit structure in which a first LC parallel resonant circuit including an inductor and a capacitor is connected to a second LC parallel resonant circuit including an inductor and a capacitor via a connecting capacitor, while the second LC bandpass filter circuit unit has a circuit structure in which a third LC parallel resonant circuit including an inductor and a capacitor is connected to a fourth LC parallel resonant circuit including an inductor and a capacitor via the other connecting capacitor; and, the common side lines of the two LC bandpass filter circuit units are connected to each other via a connecting inductor which functions as a common line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an input-output balanced filter, andmore particularly, to input-output balanced filters incorporated incommunication equipment or the like, such as cellular phones andautomobile phones, etc.

2. Description of the Related Art

In general, communication equipment such as a cellular phone, anautomobile phone, and the like, use a differential filter or aninput-output balanced filter adapted to function both as a filter and animpedance transformer, arranged between the stages of a mixer and amodulator in a transmitting circuit. As a conventional type ofinput-output balanced filter, a filter shown in the current equivalentcircuit diagram of FIG. 13 is known. The filter 101 has a circuitstructure in which an LC parallel resonant circuit 102 including aninductor L21 and a capacitor C21, and an LC parallel resonant circuit103 including an inductor L22 and a capacitor C22 are connected viaconnecting capacitors C25 and C26. In the filter 101, the signal inputbetween input terminals 111 a and 112 a is filtered and then experiencesimpedance transformation so as to be output between output terminals 111b and 112 b.

Conventionally, in forming an input-output balanced filter 101, aninductor and a capacitor, which are discrete components, are arranged tobe connected via a circuit pattern disposed on a printed circuit boardor the like. This causes the size (the occupied area) of the filter 101to be greatly increased, thereby preventing reduction in size orminiaturization of the device. Another more serious disadvantage is thatvariations in respective electric constants of the discrete componentscause a balanced transmission characteristic of the input-outputbalanced filter 101 to deteriorate significantly. Also, since theelectric constants of the discrete components are usually set by rankingof specified values, it is difficult to make fine adjustments to theelectric constants of the components so that a desirable characteristiccan be obtained. Another problem is that there are fluctuations in thecharacteristic of the filter 101 depending on the condition in which thediscrete components are mounted.

SUMMARY OF THE INVENTION

To overcome the problems described above, preferred embodiments of thepresent invention provide an input-output balanced filter, which has agreatly reduced size and, excellent and extremely stable electriccharacteristics while still allowing for very fine adjustment of theelectric characteristics of the filter.

In order to provide such an improved filter according to preferredembodiments of the present invention, an input-output balanced filterincludes a first LC bandpass filter circuit unit and a second LCbandpass filter circuit unit, in which a common side line of the firstLC bandpass filter circuit unit and a common side line of the second LCbandpass filter circuit unit are electrically connected to each othervia a common line.

The novel arrangement described above permits the approximate midpointof the common line to be a phase reference point, so that the first LCbandpass filter circuit unit and the second LC bandpass filter circuitunit have a common phase reference point. As a result, this novelarrangement controls fluctuations in a phase characteristic of theinput-output balanced filter.

Furthermore, in the input-output balanced filter according to preferredembodiments of the present invention, a layered structure is providedand includes a plurality of insulating layers, a plurality of first coilconductive patterns and first capacitor conductive patterns, a pluralityof second coil conductive patterns and second capacitor conductivepatterns, and a common line conductive pattern, in which a first LCbandpass filter circuit unit includes the first coil conductive patternsand the first capacitor conductive patterns, while a second LC bandpassfilter circuit unit includes the second coil conductive patterns and thesecond capacitor conductive patterns, and the common side line of thefirst LC bandpass filter circuit unit and the common side line of thesecond LC bandpass filter circuit unit are electrically connected toeach other via the common line conductive pattern. In this novelarrangement, the common line conductive pattern is disposed inside ofand arranged on the surface of the layered structure to have an axiallysymmetrical arrangement.

Thus, the novel arrangement described above permits the first LCbandpass filter circuit unit and the second LC bandpass filter circuitunit to be formed in a single layered structure, which allows the sizeof the filter to be greatly reduced and significantly miniaturized.Further, the electric constants of the inductors and the capacitorsdefining the first and second LC bandpass filter circuit units and theelectric characteristics of the common line are determined by geometricconfigurations and sizes of the coil conductive pattern, the capacitorconductive pattern, and the common line conductive pattern. Therefore,modifications of these conductive patterns permit design parameters ofthe filter to be freely selected, resulting in a greatly increasedfreedom of design of the filter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electric equivalent circuit diagram of an input-outputbalanced filter according to a first preferred embodiment of the presentinvention;

FIG. 2 is an exploded perspective view showing a structure of theinput-output balanced filter shown in FIG. 1;

FIG. 3 is a perspective view showing an appearance of the input-outputbalanced filter shown in FIG. 2;

FIG. 4 is a perspective view showing a first modification of a commonline conductive pattern shown in FIG. 2;

FIG. 5 is a perspective view showing a second modification of a commonline conductive pattern shown in FIG. 2;

FIG. 6 is a perspective view showing a third modification of a commonline conductive pattern shown in FIG. 2;

FIG. 7 is a perspective view showing a fourth modification of a commonline conductive pattern shown in FIG. 2;

FIG. 8 is a graph indicating attenuation characteristics of the filterwhen the common line conductive pattern is modified;

FIG. 9 is an electric equivalent circuit diagram of an input-outputbalanced filter according to a second preferred embodiment of thepresent invention;

FIG. 10 is an exploded perspective view showing a structure of theinput-output balanced filter shown in FIG. 9;

FIG. 11 is a perspective view showing an appearance of the input-outputbalanced filter shown in FIG. 10.

FIG. 12 is a perspective view showing a third preferred embodiment ofthe input-output balanced filter according to the present invention; and

FIG. 13 is an electric equivalent circuit diagram of a conventional typeof an input-output balanced filter.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to the drawings attached hereto, a description will begiven of an input-output balanced filter according to preferredembodiments of the present invention. The preferred embodiments havecommon reference numerals for the same components and portions.

FIG. 1 shows a circuit structure of a first preferred embodiment of aninput-output balanced filter according to the present invention. Theinput-output balanced filter 1 preferably has at least two LC bandpassfilter circuit units 15 and 16, which function as band-pass filters. TheLC bandpass filter circuit unit 15 has a circuit structure in which a LCparallel resonant circuit 2 including an inductor L1 and a capacitor C1and a LC parallel resonant circuit 3 including an inductor L2 and acapacitor C2 are connected to each other via a connecting capacitor C5,while the LC filter circuit unit 16 has a circuit structure in which aLC parallel resonant circuit 4 including an inductor L3 and a capacitorC3 and a LC parallel resonant circuit 5 including an inductor L4 and acapacitor C4 are connected to each other via a connecting capacitor C6.

In an input-output balanced filter, the central frequency of the LCbandpass filter circuit unit 15 is designed to be the same as that ofthe LC bandpass filter circuit unit 16.

An input terminal 11 a of the LC bandpass filter circuit unit 15 isconnected to the approximate midpoint (a center tap) of the inductor L1of the LC parallel resonant circuit 2, while an output terminal 11 b ofthe LC bandpass filter circuit unit 15 is connected to the approximatemidpoint (a center tap) of the inductor L2 of the LC parallel resonantcircuit 3. Similarly, an input terminal 12 a of the LC bandpass filtercircuit unit 16 is connected to an approximate midpoint (a center tap)of the inductor L3 of the LC parallel resonant circuit 4, while anoutput terminal 12 b of the LC bandpass filter circuit unit 16 isconnected to the approximate midpoint (a center tap) of the inductor L4of the LC parallel resonant circuit 5. Also, a common side line 8 of theLC bandpass filter circuit unit 15 and a common side line 9 of the LCbandpass filter circuit unit 16 are connected to each other via aconnecting inductor L5.

FIG. 2 shows a detailed structure of the input-output balanced filter 1,which is preferably a surface-mount type having the circuit structureshown in FIG. 1. The inductors L1 and L2 of the LC parallel resonantcircuits 2 and 3, which define the LC bandpass filter circuit unit 15,and the inductors L3 and L4 of the LC parallel resonant circuits 4 and 5which define the LC bandpass filter circuit unit 16 include coilconductive patterns 31 a to 31 e, 32 a to 32 e, 33 a to 33 e, and 34 ato 34 e, which are disposed on insulating ceramic sheets 21. Thecapacitors C1 and C2 of the LC parallel resonant circuits 2 and 3, andthe capacitors C3 and C4 of the LC parallel resonant circuits 4 and 5,and the connecting capacitors C5 and C6 include capacitor conductivepatterns 39 to 46, which are disposed on insulating ceramic sheets 23.

The coil conductive patterns 31 a to 31 e and 32 a to 32 e arerespectively located on the left side on the ceramic sheets 21. The coilconductive patterns 31 a to 31 e and 32 a to 32 e are electricallyconnected in series to each other through via holes 36 made in theceramic sheets 21 so as to define respectively the spiral inductors L1and L2 in the LC bandpass filter circuit unit 15.

The coil conductive patterns 33 a to 33 e and the coil conductivepatterns 34 a to 34 e are respectively located on the right side on theceramic sheets 21. The coil conductive patterns 33 a to 33 e and 34 a to34 e are electrically connected in series to each other through the viaholes 36 made in the ceramic sheets 21 so as to define respectively thespiral inductors L3 and L4 in the LC bandpass filter circuit unit 16.

Each of the coil conductive patterns 31 a to 31 e defining the inductorL1 is arranged on the ceramic sheets 21 symmetrically with respect toeach of the coil conductive patterns 32 a to 32 e defining the inductorL2, and with respect to each of the coil conductive patterns 33 a to 33e defining the inductor L3. In a similar manner, each of the coilconductive patterns 34 a to 34 e defining the inductor L4 is arranged onthe ceramic sheets 21 symmetrically with respect to each of the coilconductive patterns 32 a to 32 e defining the inductor L2, and withrespect to each of the coil conductive patterns 33 a to 33 e definingthe inductor L3. As a result, the winding direction of the inductor L1is reverse relative to the winding directions of the inductors L2 andL3, while the winding direction of the inductor L1 is in the samedirection as that of the inductor L4. In other words, mutually adjacentinductors have a winding direction (reverse direction) arranged tocooperatively intensify the magnetic field, while the inductorsdiagonally opposing each other on the sheets 21 have a winding direction(same direction) arranged to cooperatively weaken the magnetic field.This greatly enhances the balanced transmission characteristic of thefilter 1.

Capacitor conductive patterns 39, 41, 42, and 45 are respectivelylocated on the left side on the surface of the ceramic sheets 23. Thecapacitor conductive patterns 39 and 41 define a capacitor C1 of the LCbandpass filter circuit unit 15; the capacitor conductive patterns 39and 42 define a capacitor C2; and the capacitor conductive patterns 41,42, and 45 define a connecting capacitor C5. In a similar manner,capacitor conductive patterns 40, 43, 44, and 46 are respectivelylocated on the right side of the ceramic sheets 23. The capacitorconductive patterns 40 and 43 define a capacitor C3 of the LC bandpassfilter circuit unit 16; the capacitor conductive patterns 40 and 44define a capacitor C4; and the capacitor conductive patterns 43, 44, and46 define a connecting capacitor C6. The pairs of capacitor conductivepatterns including the pair of 39 and 40, the pair of 41 and 42, thepair of 43 and 44, and the pair of 45 and 46, are arranged to be axiallysymmetric relative to each other on the ceramic sheets 23.

An enlargement of the areas of the capacitor conductive patterns 39 to44 and an increase in the electric capacitances of the capacitors C1 toC4 allow the center frequencies of the LC bandpass filter circuit units15 and 16 to be reduced. Furthermore, an enlargement of the areas of thecapacitor conductive patterns 45 and 46 and an increase in the electriccapacitances of the connecting capacitors C5 and C6 allow a pass bandwidth of the LC bandpass filter circuit units 15 and 16 to be increased.

A common line conductive pattern 38 is disposed on a ceramic sheet 22.The common line conductive pattern 38, which has a substantiallyU-shaped configuration and an axially symmetric pattern, constitutes aconnecting inductor L5, which is a common line. Also, each of theceramic sheets 21, 22, and 23, etc., preferably is a sheet in whichdielectric particles and magnetic particles are mixed with a bindingagent, etc. The conductive patterns 31 a–34 e, 38–46 are preferably madeof Ag, Pd, Ag—Pd, Ni, and Cu, etc., and formed by printing, or the like.

The above-described ceramic sheets 21, 22, and 23 are laminated with aprotection ceramic sheet 25 and a dummy ceramic sheet 24 disposedbetween them so as to be formed into a layered structure 50 by sinteringto form an integral unit. The input terminal 11 a and a relay terminal51 of the LC bandpass filter circuit unit 15, and the input terminal 12a and a relay terminal 52 of the LC bandpass filter circuit unit 16 areprovided on the front side of the layered structure 50. The outputterminal 11 b and a relay terminal 53 of the LC bandpass filter circuitunit 15, and the output terminal 12 b and a relay terminal 54 of the LCbandpass filter circuit unit 16 are provided on the back side of thelayered structure 50. Relay terminals 55 and 56 of the LC bandpassfilter circuit unit 15 are provided on the left side of the layeredstructure 50 and relay terminals 57 and 58 of the LC bandpass filtercircuit unit 16 are provided on the right side of the layered structure50.

The input terminal 11 a of the LC bandpass filter circuit unit 15 isconnected to an extension which extends from an approximate midpoint ofthe coil conductive pattern 31 b, while the output terminal 11 b isconnected to an extension which extends from an approximate midpoint ofthe coil conductive pattern 32 b. The input terminal 12 a of the LCbandpass filter circuit unit 16 is connected to an extension whichextends from an approximate midpoint of the coil conductive pattern 33b, while the output terminal 12 b is connected to an extension whichextends from an approximate midpoint of the coil conductive pattern 34b. In other words, the input terminals 11 a and 12 a, and the outputterminals 11 b and 12 b are respectively connected to the approximatemidpoints of the inductors L1, L2, L3, and L4 for the purpose ofmatching impedance between them and the external circuit, so as tolocate center taps. Usually, the inductors L1 to L4 are set at highimpedance in advance, and the locations of the center taps are set atapproximate midpoints of the inductors L1 to L4. Moving the locations ofthe center taps permits a value of input-output impedance of the filter1 to be changed to a desired one. More specifically, when the locationsof the center taps are moved onto the capacitor conductive patterns 41to 44, the input-output impedance of the filter 1 is reduced.

The relay terminal 51 of the LC bandpass filter circuit unit 15 isconnected to an end of the inductor L1 (namely, an end of the coilconductive pattern 31 e) and an end of the capacitor C1 (namely, an edge39 a of the capacitor conductive pattern 39). The relay terminal 55 isconnected to the other end of the inductor L1 (namely, an end of thecoil conductive pattern 31 a) and the other edge of the capacitor C1 andan edge of the connecting capacitor C5 (namely, an edge of the capacitorconductive pattern 41). The relay terminal 56 is connected to an end ofthe inductor L2 (namely, an end of the coil conductive pattern 32 a), anedge of the capacitor C2 and the other edge of the connecting capacitorC5 (namely, an edge of the capacitor conductive pattern 42). The relayterminal 53 is connected to the other end of the inductor L2 (namely, anend of the coil conductive pattern 32 e), an end of the connectinginductor L5 (namely, an end 38 a of the common line conductive pattern38), and the other edge of the capacitor C2 (namely, the other edge 39 bof the capacitor conductive pattern 39).

Similarly, the relay terminal 52 of the LC bandpass filter circuit unit16 is connected to an end of the inductor L3 (namely, an end of the coilconductive pattern 33 e) and an edge of the capacitor C3 (namely, anedge 40 a of the capacitor conductive pattern 40). The relay terminal 57is connected to the other end of the inductor L3 (namely, an end of thecoil conductive pattern 33 a), the other edge of the capacitor C3, andan end of the connecting capacitor C6 (namely, an edge of the capacitorconductive pattern 43). The relay terminal 58 is connected to an end ofthe inductor L4 (namely, an end of the coil conductive pattern 34 a), anedge of the capacitor C4, and the other edge of the connecting capacitorC6 (namely, an edge of the capacitor conductive pattern 44). The relayterminal 54 is connected to the other end of the inductor L4 (namely, anend of the coil conductive pattern 34 e), the other end of theconnecting inductor L5 (namely, the other end 38 b of the common lineconductive pattern 38), and the other edge of the capacitor C2 (namely,the other edge 40 b of the capacitor conductive pattern 40).

The common side line 8 of the LC bandpass filter circuit unit 15 isdefined by the relay terminals 51 and 53, and the capacitor conductivepattern 39, while the common side line 9 of the LC bandpass filtercircuit unit 16 is defined by the relay terminals 52 and 54, and thecapacitor conductive pattern 40. The common lines 8 and 9 areelectrically connected to each other via the common line conductivepattern 38.

In the input-output balanced filter 1 having such a novel arrangement,the approximate midpoint of the common line conductive pattern 38 is aphase reference point of each of the LC bandpass filter circuit units 15and 16. Thus, since the LC bandpass filter circuit units 15 and 16 havea common phase reference point, fluctuations in the phase characteristicof the filter 1 can be suppressed. Moreover, modifications of thegeometric shape and size of the common line conductive pattern 38 allowa pole position in the attenuation characteristic of the filter 1 to beshifted. For instance, making a modification to the shapes of commonline conductive patterns 38A and 38B to provide convex shapes as shownin FIGS. 4 and 5, or making a modification to the shapes of common lineconductive patterns 38C and 38D to provide substantially H-shapedmembers as shown in FIGS. 6 and 7, allows the pole position to beshifted as shown in FIG. 8. In FIG. 8, a curve A1 indicates theattenuation characteristic of the filter 1 having the common lineconductive pattern 38 shown in FIG. 2; a curve A2 indicates theattenuation characteristic of the filter having the common lineconductive pattern 38A shown in FIG. 4; a curve A3 indicates theattenuation characteristic of the filter having the common lineconductive pattern 38B shown in FIG. 5; a curve A4 indicates theattenuation characteristic of the filter having the common lineconductive pattern 38C shown in FIG. 6; and a curve A5 indicates theattenuation characteristic of the filter having the common lineconductive pattern 38D shown in FIG. 7.

Since the inductors L1 to L5 and the capacitors C1 to C6 constitutingthe LC bandpass filter circuit units 15 and 16 are constructed to haveintegral unit layered structure 50, a compact input-output balancedfilter 1 which occupies substantially less area when mounted on aprinted circuit board is obtained. Moreover, since the ambienttemperature with respect to the LC bandpass filter circuit units 15 and16, and the operational conditions in a mounted state of the LC bandpassfilter circuit units 15 and 16 are substantially equal, an input-outputbalanced filter 1 having a stable characteristic can be obtained. Also,each electric constant of the inductors L1 to L4 and the capacitors C1to C6 defining the LC bandpass filter circuit units 15 and 16, and theelectric characteristic of the connecting inductor L5 are determined bythe geometric shapes and sizes of the coil conductive patterns 31 a to34 e, the capacitor conductive patterns 39 to 46, and the common lineconductive pattern 38. Therefore, only modifications of the conductivepatterns 31 a to 34 e, and the like, permit a free selection of thedesign parameters of the filter 1 and easy changes to the design of thefilter.

Furthermore, since the conductive patterns 31 a–32 e, 39, 41, 42, and45, which define the LC bandpass filter circuit unit 15, and theconductive patterns 33 a–34 e, 40, 43, 44, and 46, which define the LCbandpass filter circuit unit 16, are juxtaposed on the sheets 21 and 23so as to be arranged in an axially symmetric manner, the conditions forproducing the conductive patterns 31 a to 34 e, etc., are equal.Consequently, the obtained resonant circuit constants of the LC bandpassfilter circuit units 15 and 16 are substantially equal, and moreover,since the common line conductive pattern 38 has an axially symmetricarrangement, an input-output balanced filter 1 having excellent, stableand well balanced transmission characteristics is obtained.

FIG. 9 shows a circuit structure of an input-output balanced filter of asecond preferred embodiment according to the present invention; FIG. 10shows an exploded perspective view of the detailed structure of thesame; and FIG. 11 shows the appearance of the perspective view of thesame. The input-output balanced filter 61 is formed in such a mannerthat input and output are performed by the capacitors C7, C8, C9, andC10, instead of performing input and output via center taps as in theinput-output balanced filter 1 of the first preferred embodimentdescribed above.

The capacitors C7–C10 for performing input and output include capacitorconductive patterns 41 a to 44 a, 41 b to 44 b, and 71 to 74 on theceramic sheets 23, as shown in FIG. 10. More specifically, the capacitorconductive patterns 71, 41 a, and 41 b define the capacitor C7, thecapacitor conductive patterns 72, 42 a, and 42 b define the capacitorC8, the capacitor conductive patterns 73, 43 a, and 43 b define thecapacitor C9, and the capacitor conductive patterns 74, 44 a, and 44 bdefine the capacitor C10.

The edge of the capacitor conductive pattern 71 is connected to theinput terminal 11 a; the edge of the capacitor conductive pattern 72 isconnected to the output terminal 11 b; the edge of the capacitorconductive pattern 73 is connected to the input terminal 12 a; the edgeof the capacitor conductive pattern 74 is connected to the outputterminal 12 b; the edges of the capacitor conductive patterns 41 a and41 b are connected to the relay terminal 55; the edges of the capacitorconductive patterns 42 a and 42 b are connected to the relay terminal56; the edges of the capacitor conductive patterns 43 a and 43 b areconnected to the relay terminal 57; and the edges of the capacitorconductive patterns 44 a and 44 b are connected to the relay terminal58. This novel arrangement permits impedance on the input and outputsides of the filter 61 to be reduced, leading to an increase in thefreedom in design of the input-output impedance.

FIG. 12 shows a perspective view of an input-output balanced filter of athird preferred embodiment according to the present invention. In theinput-output balanced filter 81, the common line conductive pattern 38used in the input-output balanced filter of the first preferredembodiment is eliminated, and as an alternative to that, a common lineconductive pattern 82 is disposed on the side surface of the layeredstructure 50. The relay terminals 53 and 54 are electrically connectedto each other via the common line conductive pattern 82 with almost nofunction as an inductor.

In the input-output balanced filter 81 having such an arrangement, theapproximate midpoint of the common line conductive pattern 82 is thephase reference point of each of the LC bandpass filter circuit units 15and 16. Thus, since the LC bandpass filter circuit units 15 and 16 havea common phase reference point, fluctuations in the phase characteristicof the filter 81 are significantly suppressed. In addition, if thecommon line conductive pattern 82 has a function as an inductor, apartial cutting-off of the common line conductive pattern 82 by means oftrimming via a laser or the like permits fine adjustments of thecharacteristic of the filter even after commercialization of theproduct.

The input-output balanced filter according to the present invention isnot restricted to the preferred embodiments above, and can be modifiedin various forms within the scope of the invention.

When impedance between the external circuit connected to the input sideof the filter and the external circuit connected to the output side ofthe filter is different, for example, in order to perform input of thefilter, a center tap may be used, while a capacitor may be used in orderto perform output.

Further, in the above embodiments, although the LC bandpass filtercircuit units are constructed by connecting the LC parallel resonantcircuits through the connecting capacitor, the LC parallel resonantcircuits may be electromagnetically connected.

For example, they may be connected by both a connecting inductor and aconnecting capacitor. Further, conductive patterns can be omitted, andthe LC bandpass filter circuit units are not restricted to themulti-staged LC parallel resonant circuits.

Furthermore, in the preferred embodiments described above, the sheetsare laminated to be sintered in an integrated manner, but other methodsare also applicable. For example, sheets which have been sintered inadvance can be used. Also, the filter can be produced by a methoddescribed as follows: an insulating material in the form of a paste isapplied via a printing method, or the like, then the material is driedto form an insulating film, and a conductive material in the form of apaste is applied on the surface of the insulating film and dried so asto form a coil conductive pattern and a capacitor conductive pattern.Such a method of laminating sheets one by one can create a filter with alayered structure.

As clearly seen from the description above, in the present invention,the approximate midpoint of the common line defines each phase referencepoint of the first and second LC bandpass filter circuit units. As aresult, the two filters have a common phase reference point, so that aninput-output balanced filter having excellent, stable andreduced-fluctuation phase characteristics is obtained. Moreover,modifications of the geometric shape and size of the common lineconductive pattern 38 permit the pole position in the attenuationcharacteristic of the filter 1 to be easily shifted.

In addition, since the first and second LC bandpass filter circuit unitsare contained in the single layered structure, it is possible to obtaina compact input-output balanced filter which occupies significantly lessarea when mounted on a printed circuit board. Also, since the ambienttemperature with respect to the first and second LC bandpass filtercircuit units, and the operational conditions in a mounted state ofthese LC bandpass filter circuit units are substantially equal, aninput-output balanced filter having a stable characteristic can beobtained. Furthermore, the electric constants of the inductors and thecapacitors defining the first and second LC bandpass filter circuitunits and the electric characteristics of the common line are determinedby geometric configurations and sizes of the coil conductive pattern,the capacitor conductive pattern, and a common line conductive pattern.Therefore, only modifications of these conductive patterns permitsdesign parameters of the filter to be freely selected, resulting infacilitation of changes in design of the filter.

Furthermore, since the conductive patterns, which respectively definethe first LC bandpass filter circuit unit and the second LC bandpassfilter circuit unit, are juxtaposed on the insulating layer so as to bearranged in an axially symmetric manner, the conditions for producingthe conductive patterns become equal. Consequently, the resultingresonant circuit constants of the first and second LC bandpass filtercircuit units are substantially equal, and moreover, since the commonline conductive pattern has an axially symmetric arrangement, aninput-output balanced filter having excellent, stable and well-balancedtransmission characteristics is obtained.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that the forgoing and other changes in form anddetails may be made therein without departing from the spirit of theinvention.

1. An input-output balanced filter comprising: first and second inputterminals and first and second output terminals; a first LC filtercircuit unit including a common side line, said first LC filter circuitunit being connected between said first input terminal and said firstoutput terminal; a second LC filter circuit unit including a common sideline, said second LC filter circuit unit being connected between saidsecond input terminal and said second output terminal; a common linedefined by an element that is independent of said first LC filtercircuit unit and said second LC filter circuit unit; wherein said commonside line of said first LC filter circuit unit is electrically anddirectly connected to said common side line of said second LC filtercircuit unit via said common line; an approximate midpoint of saidcommon line is defined as a common phase reference point of each of saidfirst and second LC filter circuit units; and at least one of the firstLC filter circuit unit and the second LC filter circuit unit includestwo resonant portions connected via a capacitor.
 2. The input-outputbalanced filter according to claim 1, wherein the first LC filtercircuit unit includes at least one LC parallel resonant circuit.
 3. Theinput-output balanced filter according to claim 2, wherein the at leastone LC parallel resonant circuit includes an inductor and a capacitor.4. The input-output balanced filter according to claim 1, wherein thefirst LC filter circuit unit includes at least two LC parallel resonantcircuits.
 5. The input-output balanced filter according to claim 1,wherein the second LC filter circuit includes at least one LC parallelresonant circuit.
 6. The input-output balanced filter according to claim5, wherein the at least one LC parallel resonant circuit includes aninductor and a capacitor.
 7. The input-output balanced filter accordingto claim 1, wherein the second LC filter circuit unit includes at leasttwo parallel resonant circuits.
 8. The input-output balanced filteraccording to claim 1, wherein said common line includes at least oneinductor.
 9. The input-output balanced filter according to claim 1,wherein said filter has a layered unit structure and said common line isdisposed inside of said layered unit structure.
 10. The input-outputbalanced filter according to claim 1, wherein said filter has a layeredunit structure and said common line is disposed on a surface of saidlayered unit structure.
 11. An input-output balanced filter comprising:a plurality of insulating layers; first and second input terminals andfirst and second output terminals; a first LC filter circuit unitconnected between said first input terminal and said first outputterminal and having a plurality of first coil conductive patterns, firstcapacitor conductive patterns and a common side line; a second LC filtercircuit unit connected between said second input terminal and saidsecond output terminal and having a plurality of second coil conductivepatterns, second capacitor conductive patterns and a common side line;and a common line conductive pattern defined by an element that isindependent of said first LC filter circuit unit and said second LCfilter circuit unit; wherein said common side line of said LC filtercircuit unit is electrically and directly connected to said common sideline of said second LC filter circuit unit via said common lineconductive pattern; an approximate midpoint of said common line isdefined as a common phase reference point of each of said first andsecond LC filter circuit units; and at least one of the first LC filtercircuit unit and the second LC filter circuit unit includes two resonantportions connected via a capacitor.
 12. The input-output balanced filteraccording to claim 11, wherein the first LC filter circuit unit includesat least one LC parallel resonant circuit.
 13. The input-output balancedfilter according to claim 12, wherein the at least one LC parallelresonant circuit includes an inductor and a capacitor.
 14. Theinput-output balanced filter according to claim 11, wherein the first LCfilter circuit unit includes at least two LC parallel resonant circuits.15. The input-output balanced filter according to claim 11, wherein theat least one LC parallel resonant circuit includes an inductor and acapacitor.
 16. The input-output balanced filter according to claim 15,wherein the second LC filter circuit unit includes at least two parallelresonant circuits.
 17. The input-output balanced filter according toclaim 11, wherein said common line includes at least one inductor. 18.An input-output balanced filter according to claim 11, wherein saidfilter has a layered unit structure and said common line conductivepattern is disposed inside of said layered unit structure.
 19. Aninput-output balanced filter according to claim 11, wherein said filterhas a layered unit structure and said common line conductive pattern isdisposed on a surface of said layered unit structure.
 20. Aninput-output balanced filter according to claim 11, wherein said commonline conductive pattern has an axially symmetric pattern.
 21. Aninput-output balanced filter comprising: a first LC bandpass filtercircuit unit including a plurality of LC parallel resonant circuitselectromagnetically connected to one another; a second bandpass filtercircuit unit including a plurality of LC parallel resonant circuitselectromagnetically connected to one another; an inductor defined by anelement that is independent of said first LC filter circuit unit andsaid second LC filter circuit unit for electrically and directlyconnecting a common side line of the first LC bandpass filter circuitunit to a common side line of the second LC bandpass filter circuitunit; first and second input terminals provided with one of the LCparallel resonant circuits of the first LC bandpass filter circuit unitand one of the LC parallel resonant circuits of the second LC bandpassfilter circuit unit, respectively; first and second output terminalsprovided with another of the LC parallel resonant circuits of the firstLC bandpass filter circuit unit and another of the LC parallel resonantcircuits of the second LC bandpass filter circuit unit, respectively;wherein an approximate midpoint of the common line is defined as acommon phase reference point of each of the first and second LC bandpassfilter circuit units; and at least two of the plurality of LC parallelresonant circuits of at least one of said first LC bandpass filtercircuit unit and said second LC bandpass filter circuit unit areconnected via a capacitor.